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What is a Moon?

Exploring Some of the Most Significant Objects in Our Solar System.


Maurizio Cigognetti/ Photographer's choice/ Getty Images

What is a moon? It's a simple question that may, on the surface, have an obvious answer. One has to do little more than look up in the night sky on clear evenings to see an example of such an object. But when it comes to defining these magnificent objects, the answer may not be so clear.

That Bright Ball in the Night Sky

The first moon ever discovered was, unsurprisingly, our Moon. Though, it was originally thought to be a planet.

This is an artifact of the geocentric model of the solar system, essentially the belief that the Earth is the center of it all with every thing else in concentric orbit.

Our Moon actually shares much in common with planets in general, but there is one important distinction; namely that it orbits a body other than our Sun. But while our Moon is the most readily available, it is far from the only one in the solar system.

Definition of a Moon

There is actually no strict definition of what a moon is, but there are some commonalities between those objects considered moons, also called satellites. They all are:

  • Distinct, whole objects

  • Solid objects

  • In orbit around a more massive body (that presumably orbits a star)

But moons also come in all shapes in sizes. We tend to think of objects like our Moon that are large and round, but moons like Phobos and Deimos (the Moons of Mars) look more like small irregularly shaped asteroids.

This can cause confusion, especially with no lower mass limit, as to what is a moon and what is merely a grain of mass that is gravitationally bound by a planet's gravity. Also, the rings of the outer planets are not solid, but rather are composed of tiny bits of rock and ice. So should these tiny objects be considered moons as well?

Currently, these objects are considered solely part of the rings of the planet, and not individual moons. Though the official definition itself is a bit fuzzy, lending to it being more of an "understood" definition than an official statement about the nature of objects in a planet's rings.

Are All Moons Really Moons?

But there are objects that are currently classified as moons, that actually may be better classified as some other type of object.

The classic example raised is the moons of Mars, and similarly those similar ones that orbit the outer planets, that appear to be captured asteroids. While we call them moons, there are those that argue that a new classification of object should be created. Thereby separating the moons of the solar system into those that we readily think of -- the large round ones like our Moon -- and the small irregularly shaped ones that are probably gravitationally bound asteroids.

Perhaps the most controversial example, however, is the Pluto/Charon system. Pluto was demoted from planet status in 2006 to dwarf planet status. While its smaller companion Charon was deemed its moon.

But the step taken by the International Astronomers Union (IAU) to establish a strict planet definition has created a controversy with the system. By making a distinction between planets and dwarf planets -- essentially small worlds that don't quite have the properties needed to be planets -- it raises a question about whether Charon should also be considered a dwarf planet.

One of the few distinguishing properties of a moon is that it must orbit another object. But in this case Charon is nearly have the mass of Pluto. So rather than orbiting Pluto, they actually both orbit a point outside of Pluto's radius.

It is true that the Earth moves slightly in response to the mass of the Moon, the motion is so slight (because of Earth's significantly larger mass) that the center of mass of the system is still within Earth's radius. This is not the case with Pluto and Charon, because they are so similar in size.

Therefore some astronomers, including yours truly, believes that the Pluto/Charon system should be classified as a dwarf binary. But it is not a commonly held position. And there will continue to be confusion and disagreement until more strict definitions are established by the IAU.

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